The early Paleogene greenhouse climate has long captured the attention of researchers. However, due to limited data and the coarse spatial resolution of general circulation model (GCM) results, it has proven difficult to investigate early Paleogene climate at the level of detail characteristic of the actual climate system. We present a high-resolution regional climate modeling study of the North American Laramide foreland. Our simulation depicts early Paleogene climate dynamics in western North America with unprecedented detail. Increased horizontal resolution improves matches between modeled temperatures and data in some basins; however, large-scale cold biases found in prior GCM simulations persist. Our study provides insight into the existence of snow and perennial ice in the Laramide highlands and northwestern Cordillera and describes the summer monsoon along the Rocky Mountain front in great detail. Monsoonal precipitation initiates in the southeastern Rockies in May, penetrates north and west as temperatures warm, and peaks at 1 cm/day along the southern and central mountain front; basins farther to the north and west are drier.

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